The present invention generally relates to surface acoustic wave (SAW) devices and more particularly to a SAW device having an improved pass-band characteristic particularly in a super high frequency band including GHz band.
Surface acoustic wave (SAW) devices are used extensively in high frequency circuits of compact radio telecommunication apparatuses including those for portable use, to form filters and resonators. Such SAW devices are generally formed on a single crystal or polycrystalline piezoelectric substrate. Among others, a single crystal substrate of LiNbO.sub.3 designated as 64.degree. Y-X cut LiNbO.sub.3 (K. Yamanouchi and K. Shibayama, J. Appl. Phys. vol.43, no.3, March 1972, pp.856) and a single crystal substrate of LiTaO.sub.3 designated as 36.degree. Y-X cut LiTaO.sub.3 are used extensively. A 64.degree. Y-X cut LiNbO.sub.3 substrate is a 64.degree. rotated Y-cut plate of a single crystal LiNbO.sub.3 in which the direction of propagation of the surface acoustic wave is set in the X-direction. On the other hand, a 36.degree. Y-X cut LiTaO.sub.3 substrate is a 36.degree. rotated Y-cut plate of a single crystal LiTaO.sub.3 in which the direction of propagation of the surface acoustic wave is set in the X-direction.
However, these optimized cut angles, used conventionally in the piezoelectric substrates of LiNbO.sub.3 or LiTaO.sub.3, provide an optimum result only when the effect of additional mass, caused by the electrodes on the substrate, is ignored. Thus, while the substrates formed with the foregoing, conventional cut angles may provide an optimized result in the SAW devices for use in a low frequency band lower than several hundred MHz where the wavelength of the excited surface acoustic wave is sufficiently long as compared with the thickness of the electrodes, the substrate may be inappropriate for GHz applications as is required in recent portable telephone systems, due to the thickness of the electrodes which can no longer be ignored in view of the reduced wavelength of the surface acoustic waves excited therein. In such a high frequency band, the effect of the mass of the electrode is conspicuous.
It is possible, in a SAW device for use in such a super high frequency band, to expand the passband of a SAW filter or to decrease a capacitance ratio r of a SAW resonator, when the thickness of the electrode on the piezoelectric substrate is increased. By doing so, the apparent electromechanical coupling coefficients are increased. However, the SAW device of such a construction raises a problem of increased bulk wave emission from the electrodes, resulting in an increased propagation loss of the surface acoustic wave. The bulk waves emitted from the electrode as such are called SSBW (surface skimming bulk wave) and the surface acoustic wave that accompanies a SSBW is called LSAW (leaky surface acoustic wave). As to the propagation loss of the LSAW in a SAW filter that uses a thick electrode film provided on a 36.degree. Y-X cut LiTaO.sub.3 substrate or on a 64.degree. Y-X cut LiNbO.sub.3 substrate, reference should be made to Plessky et al. (V. S. Plessky and C. S. Hartmann, Proc. 1993 IEEE Ultrasonics Symp., pp.1239-1242) and Edmonson et al. (P. J. Edmonson and C. K. Campbell, Proc. 1994 IEEE Ultrasonics Symp., pp.75-79).
In the conventional SAW filters designed for using a LSAW and constructed on a 36.degree. Y-X cut LiTaO.sub.3 substrate or on a 64.degree. Y-X cut LiNbO.sub.3 substrate, it is further noted that the sound velocity of the surface acoustic wave is close to the sound velocity of the bulk wave when the thickness of the electrode is small. In such a case, there appears a spurious peak in the vicinity of the passband of the SAW filter due to the bulk wave emission from the electrode. See Ueda et al. (M. Ueda et al., Proc. 1994 IEEE Ultrasonics Symp. pp.143-146).
FIG. 1 shows the spurious peaks A and B reported by Ueda et al.(op. cit), wherein the spurious peaks A and B are formed in the vicinity of the passband of the SAW filter as a result of the bulk wave emission as noted above. The result of FIG. 1 is obtained for a SAW filter that is formed on a 36.degree. Y-X cut LiTaO.sub.3 substrate and carries thereon an interdigital electrode of an Al--Cu alloy with a thickness of 0.49 .mu.m. It should be noted that the thickness of the electrode corresponds to 3% of the wavelength of the surface acoustic wave excited in the SAW device.
Referring to FIG. 1, it will be noted that the spurious peak B is located outside the passband formed in the vicinity of 330 MHz, while the spurious peak A is formed within the passband and forms an undesirable ripple therein.
As the sound velocity of a SSBW does not change with the thickness of the electrode contrary to the sound velocity of a LSAW that changes sound velocity depending upon additional mass and hence the thickness of the electrode provided on the substrate of a SAW device, the sound velocity of the LSAW decreases relatively to the sound velocity of the SSBW when the SAW device is operated in a high frequency band such as a GHz band, resulting in a shift of the passband of the SAW filter relative to the spurious peak B. Thereby, a desirable flat passband characteristic would be obtained for the SAW filter.
However, such an increase of the electrode thickness with respect to the wavelength of the surface acoustic wave leads to the problem of increased loss of the LSAW due to the emission of the SSBW as already explained. Further, such an increase of the electrode thickness results in a deterioration of the shape factor of the SAW filter. As will be explained later, the shape factor of a SAW filter represents the steepness as well as the width of the passband characteristics of the filter. More specifically, the filter characteristic becomes broad and undefined when the shape factor of the SAW filter is poor.
Further, in a SAW filter for use in a super high frequency band including GHZ band, it is necessary to secure a certain thickness for the electrode so as to reduce the resistance of the interdigital electrodes. Such a requirement of increased thickness of the electrode is contradictory with the requirement of reduced loss and improved shape factor of the SAW device.